1. Field of the Invention
The present invention relates to a catalyst used for decomposing gaseous oxide, such as air pollution gas, and more particularly, to a catalyst used for cleaning various gases such as CO.sub.2 and NOx generated by combustion, which become causes of air pollution.
2. Description of Prior Art
Carbon dioxide gas and nitrogen oxide gas which are great factors of air pollution are derived from exhaust gas produced from factories and automobiles. For example, nitrogen oxide is not only one of factors of causing asthma or cancers in a great rate, but also it has been noticed as warming gas which brings about global warming (green housing), together with carbon dioxide gas. From the viewpoint of peventing the global warming (green housing), therefore, countermeasures of removing nitrogen gas existing in the exhaust gas have been urgently desired.
In order to solve these problems, technology has been studied and researched to reduce the nitrogen oxide in the exhaust gas. The typical method which has been practised at the present time is a dry selection contact reduction process. Except this process, there have been known no-catalyst reduction process, wet direct absorption process and wet oxidization absoprtion process.
The dry selection contact reduction process is one which adds a reductant to an exhaust gas to reduce NOx by means of a catalyst. As the reducing agent, ammonium, hydrocarbon, hydrogen and carbon monoxide are used. The catalyst is selected from noble metals such as Pt or various metal oxides carried on Al.sub.2 O.sub.3 or TiO.sub.2. However, Pt is expensive, so that the actual practice thereof is difficult.
In this point, since V.sub.2 O.sub.5 -TiO.sub.2 catalyst having TiO.sub.2 as a carrier is used at relatively low temperature and excellent in catalyst activities, it has been practised.
However, even in V.sub.2 O.sub.5 catalysts, the component V thereof is not cheap and moreover, decomposition efficiency in the catalyst activities can not be satisfied.
On the other hand, the countermeasure to prevent air pollution caused by carbon dioxide gas has been recognized, so that emission control itself has only been studied at the present situation.
With respect to the recent treatment of carbon dioxide gas, recycle system of oxygen and carbon dioxide gas in the universe has been studied as the prevention of air pollution by the carbon dioxide gas is difficult.
Nowaday, plans of space stations have been proceeded on a full scale. In order to correspond to manned commission in the space for a long period, it becomes indispensable to establish technology of recycling gas and oxygen in the space station. From these circumstances, importance of studies for treating carbon dioxide gas has increasingly rised.
As shown in FIG. 4, oxygen-recycling system in the space is formed of a cycle of removal of CO.sub.2, reproduction of an adsorbent, reduction of CO.sub.2 and production of O.sub.2 (electrolyte), in relationship between O.sub.2 which a person absorbs and CO.sub.2 which he exhausts. As CO.sub.2 -reduction process in this system, BOSCH system has been researched (Universal Station Lectures published by Nippon Koku Uchu Gakkai (Japan Aeronautics and Space Association), pp. 244-245, 1985). The BOSCH system is a relatively superior method. The reaction in this system is carried out by inserting H.sub.2 and CO.sub.2 as a mixed gas in the proportion of 2:1 in reaction cells, using steel wool as a catalyst and recycling the gas between the reaction cells.
The reaction is the following. EQU CO.sub.2 +2H.sub.2 .fwdarw.C+2H.sub.2 O
However, in the above-mentioned reaction, the following side reactions are produced: EQU CO.sub.2 +C.fwdarw.2CO EQU CO.sub.2 +H.sub.2 .fwdarw.CO+H.sub.2 O EQU 2H.sub.2 +C.fwdarw.CH.sub.4.
As a result, about 20% of CO and about 30% of CH.sub.4 are produced. As apparent from this fact, the decomposition rate of carbon is very low. Furthermore, there are such problems of (1) selection of most suitable catalyst, (2) treatment of unreacted product of side production of produced carbon product and separation of outlet water and gas, and (3) security of safety against the use of hydrogen at high temperature.
In the actual treatment, the BOSCH system requires high temperature of from 500.degree. C. to 800.degree. C. in the reaction and higher consumption of thermal energy. When such a high temperature acts on an apparatus for long time, there becomes a problem with respect to the durability of the apparatus and the selection of materials of the apparatus and thus, danger becomes great in a point of dealing with hydrogen gas at high temperature.
The present invention is to solve the above-mentioned problems.